Spread Lipid Films on Liquid Surfaces (Langmuir–Blodgett Films) - Surface and Colloid Chemistry: Principles and Applications - page 73

In the case of alkanes, the distance between the molecules in the solid phase is ca.
5 Å, while it is 5-6 Å in the case of the liquid phase. The distance between molecules
in the gas phase, in general, is ca. 1000 1/3 = 10 times larger than in the liquid phase
(water: volume of 1 mol water = 18 cc; volume of 1 mol gas = 22.4 L). In fact, mono-
molecular film studies are the only direct method of obtaining such information at
the interfaces of lipids. Considering that only microgram quantities are enough for
such information, the importance of such studies becomes clearly evident.
The isotherms of two-dimensional films are also found to resemble the three-
dimensional P-V isotherms, and one can use the same classical molecular descrip-
tion as for the qualitative analyses of the various states. However, it is also obvious
that it shall not be a complete comparison between the two-dimensional and three-
dimensional structures since there are very subtle differences in these two systems,
as described in later text.
In the three-dimensional structural buildup, the molecules are in contact with
near neighbors as well as with molecules that may be 5 to 10 molecular dimensions
apart (as found from x-ray diffraction). This is apparent because in liquids there is a
long-range order up to 5-10 molecular dimensions.
On the other hand, in two-dimensional films, the state is much different. The
amphiphile molecules are oriented at the interface such that the polar groups are
pointed toward water (subphase), while the alkyl groups are oriented away from
it. This orientation gives the minimum surface energy. The structure is stabilized
through lateral interaction between
1. Alkyl-alkyl group … attraction
2. Polar group-subphase … attraction
3. Polar group-polar group … repulsion
The alkyl-alkyl group attraction arises from the van der Waals forces. The magni-
tude of van der Waals forces increases with
Increase in alkyl chain length
Decrease in distance between molecules (or when area/molecule [A] decreases)
It means that, as the alkyl chain length increases, the films Π should become more
stable, thus giving high collapse pressure Π co . This is also found from experimental
data.
Stable films are thus formed when the attraction forces are stronger than the
repulsion forces.
4.3.1.1 Self-assembly monolayers (Sams)
Many substances as found in nature (lipids) exhibit unique properties in aqueous media.
Some lipids (such as lecithins or alike), when dispersed in water, form very well-defined
assemblies, in which the alkyl part of the molecule is in close proximity to each other.
This leads to self-assembly formation with many important consequences.
Micelle formation is one of the most common SAM structures. In fact, the whole
basis of biological cell structure and function is dependent on the lipid-bilayer